1. Field of the Invention
The present invention relates generally to a node for an optical communication network, and more specifically to a loop-type optical local area network (LAN) and a loop-type optical LAN using such a node.
2. Related Background Art
In conventional loop-type optical LANs, there have been proposed apparatuses with transmission speeds of several hundred Mbps which serve as a backbone LAN of Ethernet, token ring and the like. In these products, an increase in capacity is achieved mainly by time division multiplexing (TDM), but wavelength division multiplexing (WDM) is also being studied as a means for achieving multimedia communications. As an example of the wavelength division multiplexing system, a compound-type optical LAN has been proposed by Eda et al. (see SB-6--6 Vernal Meeting of Electronics Information Communication Academy (1991)). FIG. 1 shows the system structure of this LAN, and FIG. 2 shows the structure of a node used in the LAN. This system is constructed by the combination of FDDI (fiber distributed data interface) optical LAN and wavelength division multiplexing communication in which a plurality of wavelengths are utilized and no regenerative repeating is conducted. It should be noted that the term "wavelength" as used herein also means a signal or light having such a wavelength.
A wavelength .lambda..sub.A of the multiplexed wavelengths is assigned for FDDI, and wavelengths of .lambda..sub.1 .about..lambda..sub.n are assigned for wavelength division multiplexing communication. In FIG. 1, reference numeral 14 designates a control node for performing group or centralized management and control of the wavelength division multiplexing communication, reference numerals 15-1.about.15-3 respectively designate communication nodes which do not have such function, and reference numerals 13-1.about.13.about.4 respectively designate optical fibers used as a transmission line. The wavelength .lambda..sub.A for FDDI is regeneratively repeated in each node including the control node 14 and continues to circulate in the loop (in a counterclockwise direction in FIG. 1). On the other hand, the wavelengths .lambda..sub.1 .about..lambda..sub.n for communication are caused to enter the loop by the node that transmits such wavelength and are extracted from the loop in the node that receives such wavelength. FIG. 1 shows a state in which the node 15-1 conducts the communication using the wavelength .lambda..sub.1 and the node 15-2 conducts the receiving thereof. In this communication system, the wavelength is extracted or picked out from the loop when received, so the wavelength .lambda..sub.1 exists solely in the optical fiber 13-2 between the transmitting node 15-1 and the receiving node 15-2.
The operation of the node will be described with reference to FIG. 2.
In FIG. 2, reference numeral 1-1 designates an optical demultiplexer for extracting or selecting an access-control signal of the wavelength .lambda..sub.A for FDDI, reference numeral 2 designates a tunable optical demultiplexer which can control or change an extracted wavelength, reference numerals 3-1 and 3-2 respectively designate optical multiplexers for combining a plurality of wavelengths, reference numerals 6-1 and 6-2 respectively designate opto-electric (O/E) converters for converting an optical signal to an electric signal, reference numeral 7 designates an access processing circuit for controlling the entire node concerned, reference numeral 8-1 designates an electro-optical (E/O) converter for converting an electric signal to an optical signal, reference numeral 10 designates a tunable E/O converter in which the wavelength of an optical signal to be converted from an electric signal is changeable, and reference numeral 12 designates an interface circuit for outputting a signal from a terminal equipment for performing the wavelength division multiplexing communication (not shown) into the tunable E/O converter 10 and inputting a signal from the O/E converter 6-2 in the associated node into the terminal equipment.
For simplicity of explanation, it is assumed that the node shown in FIG. 2 conducts the transmission at a wavelength .lambda..sub.1 and the receiving at a wavelength .lambda..sub.2 during wavelength division multiplexing communication and that a wavelength .lambda..sub.3 is also on the transmission line. In FIG. 2, the wavelengths transmitted through respective portions of the node are designated together with an arrow. The wavelength .lambda..sub.1 transmitted from this node is taken in by a receiving node and is not returned to the node shown in FIG. 2, so the wavelengths received by this node from the transmission line are .lambda..sub.A, .lambda..sub.2 and .lambda..sub.3. The access-control signal wavelength .lambda..sub.A is separated from the other wavelengths by the optical demultiplexer 1-1, enters the O/E converter 6-1 for time division multiplexing communication and is converted to an electric signal to be supplied to the access processing circuit 7. The remaining wavelengths .lambda..sub.2 and .lambda..sub.3 are combined with the wavelength .lambda..sub.1 carrying a signal from the tunable E/O converter 10 of the terminal equipment concerned, at the optical multiplexer 3-1.
The wavelength .lambda..sub.2 to be received by this node is separated by the tunable optical demultiplexer 2 and input into the O/E converter 6-2. On the other hand, the other wavelengths .lambda..sub.1 and .lambda..sub.3 are combined with the access-control signal wavelength .lambda..sub.A transmitted from the E/O converter 8-1, at the optical multiplexer 3-2, and are transmitted to the transmission line.
The selection wavelengths of the tunable E/O converter 10 and the tunable optical demultiplexer 2 are controlled by the access processing circuit 7. In this case, the selection wavelength of the tunable E/O converter 10 is set to .lambda..sub.1, and that of the tunable optical demultiplexer 2 is set to .lambda..sub.2. Therefore, the tunable E/O converter 10 converts an electric signal from the interface circuit 12 to an optical signal having the wavelength .lambda..sub.1, and this optical signal is combined with the other wavelengths .lambda..sub.2 and .lambda..sub.3 by the optical multiplexer 3-1, as discussed above. On the other hand, the light of the wavelength .lambda..sub.2 taken out by the tunable optical demultiplexer 2 is converted to an electric signal by the O/E converter 6-2 and this electric signal is supplied to the interface circuit 12. As mentioned above, the interface circuit 12 inputs a signal from the terminal equipment for performing the communication in the wavelength division multiplexing system into the tunable E/O converter 10 in the node and outputs a signal from the O/E converter 6-2 into this terminal equipment. The interface circuit 12 is also controlled by the access processing circuit 7.
The prior art loop type optical LAN, however, has the following drawback. In the LAN, a case occurs where a simultaneous communication is performed, that is, a signal of a certain wavelength transmitted from a certain node is received by a plurality of nodes. In the wavelength division multiplexing communication of the loop type optical LAN, a node which first receives a signal of such a wavelength takes in all the light of this wavelength, so this wavelength cannot be transmitted to following nodes. As a result, in the wavelength division multiplexing communication of the above-discussed system, only one-to-one communication can be conducted.